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Homework Help: Electric force, theory and algebra.

  1. Sep 13, 2016 #1
    1. The problem statement, all variables and given/known data
    This question has two parts.
    There is a quarter or radius R that is charged with a net force of Q. A point like charge of net charge q,is at a distance z from the center of the quarter.
    Q1: Under what condition could we use Coulomb's law to find the magnitude the force between the quarter and the charge.
    A1: My answer is when the charge on the Quarter is concentrated at its center.
    Q2: In an experiment the force was found to be given by , F= Qq/A2ε°(1 -( z/(z^2+R^2)^2/2). Where A is the area of the quarter. Apply your condition from Q1 to show this equation is consistent with Coulomb/s law.
    sorry it looks like a mess.
    2. Relevant equations

    3. The attempt at a solution
    I tried to find the limit as R approaches zero. the part in the first set of brackets becomes zero, then so does the whole expression, so I was wondering if I approached this completely wrong, or if it's a matter of properly rearranging the expression. Thank you in advance.
  2. jcsd
  3. Sep 13, 2016 #2


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    Science Advisor
    Homework Helper
    Gold Member

    It's a matter of finding the limit properly. Are you familiar with Taylor series? If so, do a Taylor expansion and see what you get. If not, there is way to get around it that I can show you.
  4. Sep 13, 2016 #3
    Thank you, this worked and reduced to coulomb's expression XDXDXD
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